Polyester resins have been used as binders in the art in order to improve low temperature toner fixability (Patent Literatures 1 and 2). In order to improve the low temperature toner fixability still further, the molecular mass and/or the glass transition temperature Tg should be lowered with respect to the resins, however, which typically leading to poor blocking resistance of toners under high temperature and high humidity conditions. Such resins are also problematic as to reduce charging capacity of developers since the toners adhere firmly to carriers, developing sleeves, etc. Moreover, the reduction of charging capacity tends to be pronounced with time in particular under high temperature and high humidity conditions or low temperature and low humidity conditions with large image areas. As such, toners and/or image forming apparatuses have been demanded that can output stably high quality images under a wide variety of operating conditions meanwhile being substantially non-problematic under usual operating conditions.
A binder containing a charge controller or a charge control agent is proposed in order to improve charging ability or charge stability and to prevent background smear (Patent Literature 3). However, the charge controller typically exhibits a low temperature fixability inferior to that of polyester resins, thus is likely to deteriorate the low temperature fixability of polyester resins. It is therefore necessary for the toner to improve the low temperature fixability still more that the charge controller should disperse uniformly into the toner and represent a sufficient charging property in less amount.
Developers are typically used in electrophotographic, electrostatic recording or electrostatic printing processes in a way that a developer firstly attaches to a photoconductor on which an electrostatic image is formed in a developing step, then the developer is transferred from the photoconductor to a recording medium such as paper in a transfer step and fixed on the recording medium in a transfer step. The developers for developing electrostatic images on the surfaces with latent images are usually two-component developers containing a carrier and a toner or one-component developers containing a magnetic or non-magnetic toner and no carrier. In the processes as regards the two-component developers, the toner particles tend to attach the carrier surface to degrade the developer, and one-sided consumption of toners decreases the toner concentration in the developers, which requires to maintain a certain ratio between toner and carrier by means of large-size developing devices. On the other hand, the apparatuses or devices have been downsized by virtue of advanced function of developing rollers as regards the one-component developers.
In recent years, automation and coloring have been popularized still further in offices, such that various graphs by means of personal computers, images taken with digital cameras, or pictorial drafts read by scanners are printed and copied on a number of papers for personal presentation, for example. Images to be output by printers typically contain a complicated configuration including solid images, line images and halftone images even in one draft, thus are demanded in various manners along with high reliability.
Conventional electrophotographic processes on the basis of one-component developers are classified into magnetic one-component developing processes by use of magnetic toners and non-magnetic one-component developing processes by use of non-magnetic toners. In the magnetic one-component developing processes, which have been recently in practical use for numerous small-size printers etc., a magnetic toner that contains a magnetic material such as magnetites is supported by a developer bearing member with a magnetic field-generating unit therein, and the toner is thin-layered by means of a layer thickness-control member and developed subsequently. However, most of the magnetic materials are of colored or black, which affording a deficiency that the coloring is difficult.
On the other hand, in the non-magnetic one-component developing processes, a toner supply roller etc. is urged to contact with a developer bearing member thereby to supply a toner on the developer bearing member that electrostatically supports the toner, which is then thin-layered by means of a layer thickness-control member and developed, by virtue of the non-magnetic property of toners. The processes may advantageously be compliant to colorizing due to the absence of color magnetic materials, and the apparatuses may be small-sized still more and of low cost due to the absence of magnets in developer bearing members, thus have been recently in practical use for small-size full-color printers etc.
The two-component developing systems may maintain stably the charging ability and the transportability even under prolonged usage and be easily compliant with high-speed developing devices, since a carrier is employed as a means for charging and transporting, the toner and the carrier is sufficiently stirred inside a developing unit and then transported to a developer bearing member before the developing.
In contrast, there remain currently many problems to be solved in the one-component developing processes. That is, problems in charging or transporting tend to occur under prolonged usage or high speed in the one-component developing processes due to the absence of the charging and transporting means such as carriers. Specifically, when the toner is transported on the developer bearing member followed by thin-layering the toner by means of the layer thickness-control member before the developing in the one-component developing processes, toners of low or inverse charging tend to generate in a rate more than that of the two-component developing processes since the contacting or the frictional charging period is significantly shorter between the toner and the developer bearing member, the layer thickness-control member or the frictional electrification.
In non-magnetic one-component developing processes, toners or developers are transported typically by at least one toner transporting member and electrostatic latent images on the latent image are developed by use of the transported toner. In the processes, the layer thickness of the toner should be as thin as possible on the surface of the toner transporting member. This is applicable to two-component developers with carriers having a very small diameter. When one-component developers and toners with a high electric resistance are employed together with, the layer thickness of the toner should also be as thin as possible in particular, since the toners are to be charged by developing units. In cases where the toner layer is thick, the toner layer is likely to be charged at only around its surface and far from uniformly charging over the entire toner layer. Therefore, toners are required to exhibit a rapid charging velocity and an appropriate charging amount.
As such, charge control agents and additives are conventionally added to toners in order to stabilize the charging ability. The charge control agent controls and maintains the frictional charge amount of toners. The charge control agents of negative electricity are exemplified by mono azo dyes; metal salts of salicylic acid, naphthoic acid and dicarboxylic acids; metal complex salts of dicarboxylic acids; diazo compounds; and boron complex compounds. The charge control agents of positive electricity are exemplified by quaternary ammonium salts, imidazole compounds, nigrosines and azine dyes.
However, some of these charge control agents are of chromatic color and inadequate for color toners. In addition, some of these charge control agents have a poor compatibility with binder resins and those on toner surface, which mostly contributing to the charging, tend to separate from the surface and fluctuate the charging ability of toners, or may disadvantageously smear developing sleeves or cause filming on photoconductors.
Therefore, there conventionally arises a troublesome phenomenon that initial appropriate images degrade gradually to cause background smear or unclearness. In cases of continuous color copy along with supplying toners in particular, long term usage cannot be achieved since the charge amount of toners decreases and the initial tone of images significantly alters, such that no more than several thousand sheets of copy bring about premature exchange of process cartridges of an imaging unit, which leading to a large environmental load and bothersome processing of users. Moreover, heavy metals in almost all process cartridges are causing a social safety issue in recent years.
In order to solve the problems described above, resin charge-control agents are proposed that improve the compatibility with binder resins, clarity of fixed toner images and environmental safety. The resin charge-control agents may afford stable charging ability/clarity due to appropriate compatibility with binder resins. However, the charge control agents are inferior in the charge amount/charging rate compared to toners containing mono azo dyes, metal salts or metal complex salts of salicylic acid, naphthoic acid or dicarboxylic acids. When the added amount of the resin charge-control agent increases, the charging ability may be improved but the toner fixability such as low temperature fixability or offset resistance is likely to degrade. Moreover, these compounds tend to exhibit excessively large environmental stability or moisture resistance with respect to their charge amount, which possibly resulting in background smear or fog (Patent Literatures 4 to 7).
As such, copolymers are proposed that are proposed from monomers having an organic acid salt such as a sulfonic acid salt group and aromatic monomers having an electron attracting group. However, these copolymers represent an insufficient dispersion into the binder resins, and the effects on suppressing the fluctuation of toner charge amount or preventing the filming on developing sleeves or photoconductors are insufficient as regarding a prolonged period, although the charge amounts are sufficient by virtue of the moisture absorbability and tackiness derived possibly from monomers containing the organic acid salt such as the sulfonic acid salt group (Patent Literatures 8 to 11).
In addition, such copolymers are proposed, formed of monomers containing an organic acid salt like a sulfonic acid salt group, aromatic monomers containing an electron-attracting group, and styrene or polyester monomers, in order to enhance the compatibility with binder resins such as styrene resins and polyester resins, however, providing insufficient effects on maintaining the charge amount or preventing the filming on developing sleeves or photoconductors. In particular, the charge control agents are typically unsatisfactory in combination with polyester or polyol resins as used for a color toner binder resin that are usually desirable in terms of coloring property and intensity.
There have been such a technical trend that the apparatuses are small-sized, high-speed, and cost-lowered along with the printer market expanding; and currently, the apparatuses are demanded for higher reliability and longer life, toners are required to maintain their properties for a long period; however, the resin charge-control agents are less likely to maintain their charge control effect thus to blur or foul the developing sleeves or layer thickness-control members such as blades and rollers, consequently decreasing charging ability of toners and causing filming on photoconductors.
The small-sized, high-speed apparatuses necessarily lead to developing processes with lower amounts of developers and shorter periods, which requiring developers having an excellent initial charging property. A variety of developing systems have been proposed for both of one-component developers and two-component developers; non-magnetic one-component development is desirable for printers by virtue of small-sizing or weight-saving ability and absence of carriers. In the developing systems, the toner amount on developing rollers is adjusted by way of forcibly frictioning and attaching toners on developing rollers or by means of blades since such properties are poor as toner-supplying ability onto the developing rollers and toner-sustaining ability on the developing rollers. As a result, there arise such problems as filming tendency of toners onto the developing rollers, shorter lifetime of the developing rollers and unstable charge amount of toners, and these problems possibly disturb adequate development. Accordingly, color toners for the non-magnetic one-component development are often unsatisfactory in thermal resistance of toner binder resins in addition to usually necessary properties for conventional color toners, thus are likely to cause toner filming on the developing rollers.
Furthermore, Patent Literatures 1 to 4 describes Examples that show poor charge amount and charging velocity. When the added amount of resin charge-control agents for the countermeasure is increased, the charging ability may be improved but the toner fixability such as low temperature fixability or offset resistance is likely to be deteriorated. Moreover, these compounds tend to exhibit excessively large environmental stability or moisture resistance in their charge amount, which possibly resulting in background smear or fog.
Furthermore, the proposals in Patent Literatures 8 to 11 may assure a sufficient charge amount due to moisture absorbability or adhesive property, however, there remain such problems as insufficient dispersion into toner binders, unsatisfactory suppression of charge fluctuation and insufficient effect on preventing filming onto sleeves and photoconductors.
In forming images by electrophotographic processes, a latent image is electrostatically formed on an image bearing member of photoconductive materials etc., then charged toner particles are attached to the electrostatic latent image to form a visible image, followed by transferring the toner image onto a recording medium like papers and fixing thereof to produce an output image. In recent years, electrophotographic copiers and printers are changing rapidly from monochrome to full-color systems, and the full-color market has been expanding.
In forming color images by full-color electrophotographic processes, typically, color toners of three elementary colors of yellow, magenta and cyan or four colors adding black thereto are duplicated to reproduce every color. In order to produce clear full-color images with excellent color reproducibility, therefore, the surface of fixed toner images should be somewhat smoothed to decrease optical diffraction, and it is also important that pigments are uniformly dispersed into toners and the dispersed pigments maintain the finely dispersed condition without re-coagulating.
In order to reproduce the color of human skin in particular, it is required that the color is expressed by a subtractive mixing process through overlapping a yellow toner and a magenta toner, thus an optimum combination from yellow pigments, magenta pigments and resins for dispersion matrix has been investigated as a subject matter.
Patent Literature 12, for example, discloses a magenta toner for developing electrostatic images, in which the toner is prepared by way of dissolving a toner composition, containing a polyester resin modified to form a urea bond, into an organic solvent to form a solution, which then undergoes a polyaddition reaction, then the dispersion liquid is removed for the solvent and rinsed, and the toner contains at least a colorant of a specific compound.
In addition, Patent Literature 13 discloses a magenta toner for electrophotography containing at least a binder resin and a colorant, in which the toner contains a naphthol pigment having a certain structure as the colorant, and the tone has a shape factor SF1 of 110 to 140 and a volume average particle diameter of 2 to 9 μm.
However, these proposals may be far from recovering by themselves the poor color reproducibility due to pigment re-agglomeration in toners, and thus the color reproducibility of images is currently far from accurate reproduction as for human shin color in particular.
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 62-178278
Patent Literature 2: JP-A No. 4-313760
Patent Literature 3: JP-A No. 7-062766
Patent Literature 4: JP-A No. 63-88564
Patent Literature 5: JP-A No. 63-184762
Patent Literature 6: JP-A No. 03-56974
Patent Literature 7: JP-A No. 06-230609
Patent Literature 8: JP-A No. 08-30017
Patent Literature 9: JP-A No. 09-171271
Patent Literature 10: JP-A No. 9-211896
Patent Literature 11: JP-A No. 11-218965
Patent Literature 12: JP-A No. 2004-77664
Patent Literature 13: JP-A No. 2003-215847